CN1080279C - Improved method for recovering diacetate esters of polytetramethylene ethers - Google Patents

Abstract

An improved method for recovering pure polytetramethylene ether diesters comprising the use of a solid acid (such as Nafion®) as a catalyst and a carboxylic acid with carboxylic anhydride (such as acetic acid with acetic anhydride) as a molecular weight control agent Carrying out the polymerization reaction of tetrahydrofuran (THF) optionally with one or more comonomers (such as 3-methyltetrahydrofuran, ethylene oxide, propylene oxide, etc.), after the polymerization reaction, the product recovery including unreacted THF Bulk flashing and stripping of residual THF, carboxylic acids and carboxylic anhydrides (such as acetic acid and acetic anhydride) with superheated THF. This method can be effectively used to produce high purity technical grade PTMEA, which has the advantage of overcoming the problems associated with the presence of high boilers when converted to PTMEG.

Description

Improved method for recycling polytetramethylene ether diacetate

technical field

This invention relates to an improved process for the recovery of pure polytetramethylene ether diesters. More specifically, but not by way of limitation, the present invention relates to tetrahydrofuran (THF) with a secondary cyclic ether comonomer or no comonomer using a solid acid catalyst and carboxylic acids with carboxylic anhydrides as molecular weight control agents wherein after polymerization, product recovery includes bulk flashing of unreacted THF and stripping residual THF, carboxylic acid and carboxylic anhydride with superheated THF.

craft background

Polytetramethylene ether glycol (PTMEG) is a product in the chemical industry that is widely used to form block copolymers with multifunctional polyurethanes and polyesters. Usually PTMEG is prepared by reacting tetrahydrofuran (THF) with fluorosulfonic acid, and then quenching the product with water. While this method is quite satisfactory, it is not satisfactory because the acid cannot be recycled for reuse and waste acid disposal is a major problem due to its toxicity and corrosiveness.

US Patent No. 4,120,903 discloses THF polymerization using polymers containing α-fluorosulfonic acid groups as catalysts and water or 1,4-butanediol as chain terminators. The nature of the catalyst is such that it can be reused, thereby eliminating disposal problems, while the insolubility of the catalyst in the reaction mass allows easy separation of the catalyst from the product after polymerization. This very low solubility of the catalyst also reduces its loss during the reaction. However, the polytetramethylene ether glycol produced by this method has a molecular weight of 10,000 or more, while the commercial product generally has a molecular weight of less than 4,000, and its main part has a number average molecular weight of 650 to 3,000.

U.S. Patent No. 4,163,115 discloses the use of a fluorinated resin catalyst containing sulfonic acid groups to carry out the polymerization of THF and/or THF comonomers to prepare polybutylene ether diesters. Acyl ion precursors to adjust the molecular weight. This patent discloses the use of acetic anhydride and acetic acid with a solid acid catalyst. Polymerization product was isolated by stripping unreacted THF and acetic acid/acetic anhydride. The isolated product is polymeric tetrahydrofuran diacetate (PTMEA), which must be converted to the corresponding dihydroxy product polytetramethylene ether glycol (PTMEG), which is a raw material for most polyurethane end-use applications.

US Patent No. 5,149,862 discloses a method for preparing PTMEA by using a zirconium-based catalyst for THF polymerization. Its molecular weight can be adjusted arbitrarily by adding acetic acid and acetic anhydride. Unreacted THF, acetic acid and acetic anhydride are removed by distillation or stripping with steam or an inert gas such as nitrogen. Steam stripping, however, requires temperatures that cause degradation of the polymer, while re-separation of water from the polymer requires extended unit operations and stripping with nitrogen requires long run times and/or requires impractical amounts of nitrogen.

Residual acetic acid and acetic anhydride in PTMEA can have various adverse effects. If CaO is used as a catalyst in the successive transesterification reactions, the formation of calcium acetate with acetic acid will cause gelation of the transesterification reaction medium. If NaOMe/NaOH is used as a catalyst, acetic acid and acetic anhydride act as a neutralizing catalyst, thereby inhibiting the transesterification reaction. Residual acetic anhydride also reacts with methanol to form methyl acetate, and even low concentrations of methanol acetate can adversely affect the final level of conversion of PTMEA to PTMEG.

Especially in the diacetate purification of copolymers of THF and 3-methyltetrahydrofuran (3-MeTHF), high concentrations of acetic anhydride are encountered, so there is a need for an efficient method of stripping these high boilers.

disclosure of invention

In view of the above-mentioned problems associated with the production and recovery of polytetramethylene ether diesters, the present invention provides a method for producing polytetramethylene ether diesters by using a solid acid catalyst and a carboxylic acid having a carboxylic anhydride as Molecular weight control agent THF is optionally polymerized with one or more substituted THF or alkylene oxide comonomers in a reactor, and the improvement includes the following steps: using superheated THF to polymerize from polytetramethylene ether diester Unreacted carboxylic acid and carboxylic anhydride are stripped from the reaction product. The present invention also provides such a method, and its improvements include the following steps:

a) A solution product stream comprising polytetramethylene ether, unreacted THF and carboxylic acid with carboxylic anhydride is recovered from the reactor.

b) Flash off most of the unreacted THF from the polytetramethylene ether diester solution under reduced pressure.

c) Stripping residual THF, carboxylic acid and carboxylic anhydride using superheated THF.

In one embodiment of the invention, the carboxylic acid is acetic acid and the carboxylic anhydride is acetic anhydride.

It is a primary object of the present invention to provide the efficient use of superheated THF to elevate boilers from product streams thereby overcoming the problems associated with high temperature stripping. The achievement of this and other objects will become apparent upon a complete reading of the specification and the appended claims.

Brief description of the drawings

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram illustrating a particular embodiment of an overall improved process for preparing polytetramethylene ether diacetate according to the present invention.

Modes of Carrying Out the Invention

The overall process found by the present invention to produce polytetramethylene ether diester compositions broadly includes, as is well known in the prior art, the use of high polymers containing sulfonic acid groups capable of ring-opening polymerization of cyclic ethers, etc Any kind of polymerization reaction carried out by acid solid catalyst. By way of example and not limitation, it includes polymeric catalysts containing sulfonic acid groups and optionally carboxylic acid groups. A particularly preferred class of solid acid catalysts is one whose polymer chains are tetrafluoroethylene or chlorotrifluoroethylene and a Catalysts for perfluoroalkyl ethyl ether copolymers containing sulfonic acid group precursors (with or without carboxylic acid groups), as disclosed and illustrated in U.S. Patent Nos. 4,163,115 and 5,118,869, commercially available from E.I.duPont Corporation Catalysts available under the Nafion(R) trademark. The following description and examples refer primarily to Nafion(R), because the benefits and advantages of the improved process according to the present invention are optimized with this highly active catalyst. Although for the purposes of the present invention, other heterogeneous catalysts including catalytic sites for sulfonic acid groups as described above are considered equivalent to Nafion(R) in that one or more of the advantages and benefits of the present process are obtained or realized.

The polytetramethylene ether diester compositions produced by the effective method found in the present invention are generally any polyether known in the prior art for typical production by acid-catalyzed cyclization in the presence of a carboxylic acid and carboxylic anhydride. Prepared by ring-opening polymerization of ethers or mixtures in which tetrahydrofuran is the main and/or dominant reactant, ie a lot of THF is combined into the PTMEA product. More specifically, polyether diesters are prepared by polymerization of THF with alkyl-substituted tetrahydrofuran (preferably 3-MeTHF) or comonomer-free and from THF with 3-MeTHF (or without 3-MeTHF). -MeTHF) and alkylene oxide or equivalent comonomer copolymerization reaction. Thus, the following description and examples will primarily deal with the polymerization of THF and/or THF with 3-MeTHF with other comonomers optionally present. For purposes of describing and claiming the present invention, the term "polybutylene ether" generally includes homopolymerized THF polyether backbones as well as corresponding copolymerized polymers.

The THF used in the present invention may be any commercially available THF. The water content of THF should preferably be less than 0.001% by weight, while the peroxide content should be less than 0.002% by weight and optionally contain an oxidation inhibitor such as butylated hydroxytoluene to prevent the formation of undesirable by-products and color . If desired, 0.1% to about 50% by weight of THF can be used together with THF. Alkyl-substituted tetrahydrofurans copolymerizable with THF can be used. A particularly preferred alkyl-substituted THF is 3-MeTHF.

The solid acid catalyst used in the present invention broadly includes any high acid solid catalyst capable of ring-opening polymerization of cyclic ethers and the like as known in the prior art. By way of example and not limitation, it includes polymer catalysts containing sulfonic acid groups and optionally carboxylic acid groups or without carboxylic acid groups, highly acidified natural clays (such as acidified montmorillonite) and/or zeolites, Acidified zirconium sulfate/tin compounds, etc. Particularly preferred solid acid catalysts are perfluoroalkylethyl ethers whose polymer chains are tetrafluoroethylene or chlorotrifluoroethylene and a precursor containing sulfonic acid groups (with or without carboxylic acid groups) Copolymer catalysts such as those disclosed and illustrated in US Patent Nos. 4,163,115 and 5,118,869 are commercially available under the Nafion(R) trademark from E.I. duPont Company. The following description and examples will refer primarily to Nafion(R), since the benefits and advantages of the improved process according to the present invention are optimized with this highly active catalyst. Although for the purposes of the present invention, the above-mentioned other heterogeneous catalysts are considered to be equivalent to Nafion®, because one or more advantages and/or benefits of the process of the present invention can be obtained or realized, especially catalysts whose catalytic activity is close to Nafion®.

The carboxylic anhydrides used according to the invention together with the corresponding carboxylic acids as molecular weight control agents are generally those carboxylic acid anhydrides in which the carboxylic acid moiety contains 1 to 36 carbon atoms. Particularly preferred are those carboxylic acid anhydrides of 1 to 4 carbon atoms. Examples of these carboxylic acid anhydrides are acetic anhydride, propionic anhydride and the like. The most preferred anhydride for use in terms of efficacy is acetic anhydride, so only the most preferred combination of acetic acid and acetic anhydride will be specified in the following description and examples. When maleic anhydride is used, the conversion of PTMEG dimaleate to PTMEG is by hydrogenation rather than transesterification/methanolysis (see example in US Pat. No. 5,130,470). Acetic anhydride and acetic acid used in the method of the present invention can be any product that can be purchased on the market.

The action of carboxylic acids containing carboxylic anhydrides is currently believed to be substantially consistent with the chemical mechanism proposed and described in U.S. Patent No. 4,167,115, wherein the carboxylic anhydride is an acyl ion precursor that reacts with THF to form an acyl oxygen in the presence of THF and a solid acid catalyst. Onium ions initiate the reaction with the simultaneous formation of carboxylic acid molecules, which subsequently lead to ring-opening polymerization (ie, propagation through the oxonium ion mechanism) and final termination by reaction with carboxylic acid. Although, the present invention is not considered to depend on a simple mechanistic explanation, it is currently felt that the initial initiation stage of highly active solid acid catalysts such as Nafion® should be considered reversible and that the molecular weight is ultimately dependent on the ratio of initiation and chain termination ( i.e. mechanistically/mathematically independent of growth rate). This discovery leads to the particular advantage of being able to control the molecular weight of the PTMEA product by varying the mixture fed to the reactor. More specifically, for a given amount of catalyst, THF reactant concentration, and operating conditions, the molecular weight of the product is determined by the ratio of carboxylic acid to carboxylic anhydride. In fact, the amount or rate of carboxylic anhydride added to the reactor becomes the primary operating parameter determining molecular weight due to the use of THF flash and carboxylic acid stripping cycles according to the present invention. The present invention thus provides an easy and reliable method for controlling one of many commercially important characteristics of a product.

Reaction scheme and reactor design

According to a kind of typical whole reaction scheme of the present invention schematically represented in Fig. 1, wherein the particularly preferred embodiment of preparing polytetramethylene ether diacetate is in the presence of acetic acid (HOAc) and acetic anhydride (ACAN) Nafion® is used as a catalyst to complete the polymerization reaction of tetrahydrofuran (THF). As shown, the THF, HOAc and ACAN mixture stored in the premix tank 10 is continuously fed to a single-stage reactor system containing the solid acid Nafion, which acts as a polymerization reaction as described in U.S. Patent No. 4,163,115 Catalyst. It will be appreciated that during continuous commercial operation, the premix tank 10 may be omitted or replaced with a static mixer, etc., provided that the added streams, including the various recycle streams, are quantitatively controlled according to the criteria hereinafter described and exemplified correctly. . Accordingly, for the purposes of the present invention, these alternatives, etc., should be considered equivalent to the particular embodiment employing premixer 10 as shown.

During the passage through the reactor, the THF ring is opened and a tetrahydrofuran polymer terminated by acetate groups is generated. The temperature of the single-stage reactor 12 is controlled by adjusting the pressure so that the polymerization reaction proceeds under the condition of THF evaporation. Preferably, the reactor should be equipped with a low-pressure (eg, vacuum) reflux device, thereby recovering from the reactor the heat of reaction released by the polymerization of THF into PTMEA by top reflux (not shown) under reduced pressure.

The reaction product is a solution consisting of polytetramethylene ether glycol diacetate (PTMEA), unreacted THF, acetic anhydride, and acetic acid. The amount of THF reacted is a function determined by temperature, contact time, feed composition and catalyst. The molecular weight of PTMEA is mainly dependent on the feed composition, especially the concentration of acetic anhydride in the feed. The molecular weight of PTMEA can increase as the concentration of acetic anhydride in the reactor feed decreases. The acetic acid in the feed acts primarily to prevent runaway polymerization and secondarily to remove reactants to produce gels.

A typical conversion of about 35% was observed in the reactor, so that about 60-65% of the unconverted THF and acetic acid needed to be stripped. Accordingly, the reactor effluent is directed to THF flash unit 14, which is preferably controlled under vacuum (typically about 400 to about 450 mmHg) during the THF flash. Due to the presence of a substantial amount of unreacted THF, the heat of the polytetramethylene ether diester solution in the reactor was used at this step to accomplish bulk flashing of THF and acetic acid. The vapor phase from this integral flash unit 14 (again in the particular embodiment shown) is recycled back to the premix tank 10 before being reintroduced to the reactor 12. The flashed PTMEA product stream is sent to THF stripping unit 16 where residual THF, ACAN and HOAc are stripped in countercurrent with hot THF under reduced pressure. The vapor phase obtained from this superheated THF stripping step Also circulated back to the premix tank. Alternatively, these recycle streams may be blended with each other or with other feeds directly into stirred reactor 12.

Thus, for the purpose of the present invention, it is intentional and beneficial to operate the polymerization reactor under reduced pressure (ie under vacuum), thus producing technical grade PTMEA under THF evaporative cooling. In this process, cooling of the reaction mixture is assisted by temperature control provided by removal of evaporated THF and thus adjustment of reactor pressure. Polymerization is followed by a bulk flash of THF at about 200 to 600 mmHg, preferably at about 410 mmHg, due to the inherently limited conversion of THF resulting in a large amount of unreacted THF in the reactor effluent. Again, since a considerable amount is flashed, a large amount of thermal energy must be supplied to the reactor effluent. According to the purpose of the present invention, this heating step can use any method known and implemented in the prior art, not limited as an example, such as warming and/or heating outflow before or during flashing of the actual THF vapor phase method of logistics. The bulk flashed PTMEG product stream is then subjected to a superheated THF stripping operation. Typically, a reverse flow column with molten PTMEG flowing down is controlled under vacuum (eg, 20 mmHg at the top of the column) while THF is added at the bottom of the column [eg, 135°C @ 6.2 bar (90pisa)] for stripping. In such a process, any remaining THF and other high boilers including acetic acid and acetic anhydride are removed and recycled. Therefore, according to the improved method of the present invention, the advantages of the super-active solid catalyst such as Nafion have been obtained, and the traditional product quality problems produced by the residual high boilers in the excess heat of reaction and high-temperature stripping PTMEA product have also been overcome simultaneously.

This polymerization reaction can be carried out batchwise or continuously. However, to obtain the advantages of a highly active catalyst such as Nafion(R) while taking advantage of the heat released by the reaction, a continuous stirred tank reactor operating under evaporative reaction conditions was found to be preferred. Therefore, a PTMEA with a nominal volume of about 45.4 kg/hr (100 lbs) per hour and a continuously stirred reactor based on the schematic flow diagram of Fig. 1 were constructed and applied to the following examples (except that the jacketed reactor was used to remove The heat of reaction is not in addition to THF evaporative cooling). In this unit, a feed tank is used to premix the various components for the polymerization reaction, these components are (1) a recycle THF stream containing THF, acetic acid and a small amount of acetic anhydride, which are produced by the flash unit And the return of the superheated THF stripping device installed in the downstream of the polymerization reactor, (2) fresh THF, the addition rate is mainly determined according to the polymer yield [that is, about 45.4kg/hr (100lbs/hr)), (3) fresh THF Anhydrides are added at a flow rate determined by the molecular weight required. Acetic anhydride is consumed in large quantities during the polymerization reaction, so acetic anhydride is continuously added to the premixer, and acetic acid is not consumed during the reaction, so acetic acid is returned to the recycle stream along with THF. As mentioned above, the generation of the recycle stream is due to the limited conversion of THF to PTMEA in the polymerization reactor.

A stream of premixed THF, acetic anhydride and acetic acid of the desired composition was pumped continuously into a glass lined stirred tank reactor designed for a residence time of about 30 to about 60 minutes. The use of a lined reactor is required to exclude the possibility of deactivation of the catalyst by reaction of the catalyst's sulfonic acid functional groups with the metal. Alternatively, Teflon(R) or similar material may be used for this lining, but glass is preferred. The reactor contains between about 2% and 40% of the catalyst feed, preferably about 5% to 15%, most preferably about 10%. The catalyst is continuously suspended by stirring.

Stripping of THF/ACAN/HOAc

One of the key steps in the purification of PTMEA is the removal of acetic acid and unreacted acetic anhydride. Because both components are high boilers relative to THF, the removal of acetic anhydride and acetic acid is a problem, especially in large-scale operations. In laboratory operations, these components can be removed by continuous drying with nitrogen purge, but this operation is not suitable for large-scale operation. In addition, the purification of PTMEA so that the concentration of residual acetic acid and acetic anhydride is lower than 100 ppm also requires a temperature higher than 140° C. Unfortunately, when PTMEA is exposed to such a high temperature for a long time, it will change color and cause a certain degree of degradation.

An object of the present invention relates to the removal of acetic acid and acetic anhydride from PTMEA using a conventional stripping column, ie stripping acetic acid and acetic anhydride from the PTMEA reaction product with vaporized THF. Water cannot be used in this particular application, while THF is ideal for purifying PTMEA.

The reactants in the polymerization reactor are first heated to 90°C to 130°C under a back pressure of about 8.27 bar gauge (120 psig), and then injected into a container controlled at a vacuum of about 200mmHg to 600mmHg. In this step most of the THF/acetic anhydride/acetic acid flash off is cooled and recycled. The PTMEA and residual THF/acetic anhydride/acetic acid are then sent to a THF stripper where THF vapor is used to strip residual acetic acid and acetic anhydride. The THF stripper is a column, preferably a packed column with 3 to 10, more preferably 5 theoretical plates. Residual acetic acid and acetic anhydride were stripped from the polymer solution with superheated THF vapor. THF was heated to about 135°C. THF vapor is flashed into a stripping column controlled at about 20 mmHg. The stripped THF/acetic acid/acetic anhydride is condensed and recycled.

example

A polymer solution containing 33.4% PTMEA, 62.9% THF, 3.0% acetic acid and 0.7% acetic anhydride was heated to about 120°C and fed to a THF separator controlled at 450 mmHg. The top of the separator contained 94.9% THF, 4.1% acetic acid, and 1.0% acetic anhydride, while the remaining polymer solution contained 96.6% PTMEA, 2.3% THF, 1.0% acetic acid, and 0.1% acetic anhydride. This polymer solution was applied to the upper plate of a column packed with a stainless steel packing having 10 theoretical plates controlled at 20 mmHg. THF heated to 135°C at 5.17 bar gauge (75 psig) was fed to the bottom of the column. Superheated THF strips the remaining acetic acid and acetic anhydride from the polymer solution, so that the overhead distillate contains 94.7% THF, 4.7% acetic acid and 0.6% acetic anhydride, while the remaining polymer solution has 99.8% PTMEA, 0.2% THF And less than 100PPM of acetic acid and acetic anhydride. When exiting the line, the color of the product was less than 10 APHA units, an indicator of the effectiveness of removing acetic acid and acetic anhydride without affecting the quality of the PTMEA.

Industrial applicability

The ability to produce polymers with low color and low acetic acid concentrations is a major improvement over the prior art. In particular, control of the excellent color of the product was confirmed, even in the presence of unsaturated impurities, which are generally considered detrimental to THF polymerization, discoloring the product.

Claims (4)

1. A method for producing polytetramethylene ether diester by using a solid acid catalyst and a carboxylic acid with carboxylic acid anhydride - the carboxylic acid is a carboxylic acid with 1-4 carbon atoms - as the molecular weight Control agent Polymerize THF optionally with one or more substituted THE or alkylene oxide comonomers in a reactor, characterized in that the process comprises the steps of: using superheated THF from polytetramethylene ether The diester polymer is stripped of unreacted carboxylic acid and carboxylic anhydride. 2. The method of claim 1, wherein the carboxylic acid is acetic acid and the carboxylic anhydride is acetic anhydride. 3. The method of claim 1, further comprising the steps of: a) recovering from said reactor a product stream comprising polytetramethylene ether, unreacted THF and a solution of carboxylic acid with carboxylic anhydride, b) flash off most of the above unreacted THF from the polytetramethylene ether diester solution under reduced pressure, and c) Stripping residual THF, carboxylic acid and carboxylic anhydride with superheated THF. 4. The method of claim 3, wherein the carboxylic acid is acetic acid and the carboxylic anhydride is acetic anhydride.

Images